Elastic Behavior of Nb₂O₅/Al₂O₃ Core-Shell Nanowires in Terms of Short-Range-Order Structures

Single-crystalline niobium pentoxide nanowires (NWs) of length 10-15 μm and diameter 100-200 nm are synthesized by thermal oxidation of niobium substrates in a mild vacuum (3-10 mbar). Amorphous Al₂O₃ shells of varying thicknesses (10, 30, 40, and 50 nm) are deposited on top of the wires using atomic layer deposition. Bending tests of the uncoated Nb₂O₅ NWs and the Nb₂O₅/Al₂O₃ core-shell NWs are carried out inside a scanning electron microscope using a micromanipulator with a force measurement tip. The experimental deflection curves are modeled with Euler-Bernoulli (E-B) beam theory, and the Young's modulus is manipulated to determine the best fit. The Nb₂O₅ NWs with no shell are determined to have a Young's modulus of 67 ± 10 GPa, which agrees with the published data on Nb₂O₅ thin films. For core-shell NWs, only small deflections of the wires with 10 and 30 nm thick shells can be fitted with the E-B model when utilizing constant Young's modulus values of 67 GPa for the Nb₂O₅ core and about 160 GPa for the Al₂O₃ shell. When allowing for a change in the Young's modulus of the Al₂O₃ shell, the Young's modulus is determined to be at 120 ± 10 GPa for 10 nm and 145 ± 12 GPa for 30 nm at the highest applied load. For thicknesses of 40 nm and 50 nm, we observed a reduced but constant 120 ± 11 and 111 ± 10 GPa, respectively. Such behavior may result from structural disordering of the amorphous Al₂O₃ through reducing fractions of the densely packed polyhedra, while the fractions of the loosely packed polyhedra increase as a result of the increasing strain or the fabrication process. The increased disorder is associated with increased average interatomic spacing. Thus, the atomic bonding force and also the Young's modulus decrease.